I almost always do a test out stitch first especially if it is a free design. I don't alway test out those I get from Embroidery Library first because I've never had a problem from them. Like cutebuns said try and test out on the same type of fabric that you will be doing your final stitch out on. That way you will also know if the stabalizer you are using will work out. I have a t-shirt that I messed up on and if my final stitch out will be a tshirt I just do the design on another part of that messed up shirt. I have towels that I do the same thing with. Yes it does waste a little thread and stabalizer but in the long run it is worth it.